This invention relates to piston and piston ring assemblies for internal combustion engines. More particularly, this invention relates to piston--piston ring assemblies that reduce engine exhaust emissions.
A typical prior art internal combustion engine has a crankcase for containing oil, at least one combustion chamber, and at least one piston that is generally disposed between the crankcase and the combustion chamber, and is reciprocable in a cylinder. The outer surface of the piston typically has three piston ring grooves formed in it, with a piston ring disposed in each of the grooves.
In a typical piston--piston ring assembly, the piston rings have several purposes. The piston ring closest to the combustion chamber, generally called the upper piston ring, has a primary purpose of sealing the combustion chamber against leakage of gases to maintain compression within the combustion chamber. However, the middle piston ring also provides some compression sealing.
The primary purpose of the lowest or third piston ring--the ring closest to the crankcase--is to control the amount of oil that enters and remains in the cylinder; the lowest piston ring has an oil wiper surface that wipes excess oil from the cylinder wall.
However, the upper and middle piston rings do not prevent all combustion gases, created during the combustion process, from passing into the space between the piston and the cylinder wall and into the space between the piston rings. Some combustion gases are trapped in the space between the outer surface of the piston and around the piston rings. When the exhaust valve is opened, these combustion gases pass back into the combustion chamber, past the opened exhaust valve, into the exhaust manifold, and out to the atmosphere because the pressure on the combustion chamber side of the upper piston ring is much lower then the pressure on the crankcase side of the upper piston ring. As a result, the noxious emissions of the engine are increased. In a typical prior art engine like the one discussed above, and assuming that no catalyst is used, the engine output of hydrocarbons is between about 12 to 20 grams per horsepower per hour of operation for a single cylinder engine.
Therefore, it is desirable to reduce the level of hydrocarbons that are exhausted out of an internal combustion engine.